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Influence of composition, structure and testing atmosphere on the tribological performance of W-S-N coatings
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.ORCID iD: 0000-0002-2535-3107
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.ORCID iD: 0000-0003-3633-8487
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
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2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 258, 86-94 p.Article in journal (Refereed) Published
Abstract [en]

W-S-N coatings deposited by reactive magnetron sputtering offer the possibility of ultra-low friction in unlubricated sliding. In this work, W-S-N coatings of different composition and structure have been deposited, characterised and evaluated with respect to the tribological performance and tribofilm formation. The composition was varied by changing the flow of N-2 into the deposition chamber, leading to N contents ranging from 0 to 47 at.%. W-S-N coatings deposited without substrate heating are amorphous, while substrate heating results in coatings containing nanocystalline tungsten sulphide (WSx) for low N contents, and nanocrystalline tungsten nitride (WyN) at a high N content. The coatings were tribologically tested against steel balls in four different atmospheres dry N-2, dry air, humid N-2 and humid air to study the effects of atmospheric O-2 and H2O both separately and simultaneously. In dry N-2, all coatings exhibited an excellent performance with very low friction (mu approximate to 0.02) and wear. Notably, this included the N-richest and hardest coating, containing nanocrystalline WyN and only 13 at.% of S. The friction and wear increased on changing the atmosphere, in the order of dry air-humid N-2-humid air. In these three non-inert atmospheres, the friction and wear also increased with increasing N content of the coating. It is thus concluded that the presence of O-2, the presence of H2O, and a high N content (i.e., low Wand S contents) are three factors increasing the risk of high friction and wear, especially when occurring together. Raman spectroscopy mapping of the contact surfaces on the coatings and the balls showed that low friction and wear is connected to the presence of WS2 tribofilms in the contact, and that the three previously mentioned factors affect the formation and function of this tribofilm.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 258, 86-94 p.
National Category
Tribology Inorganic Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry; Engineering Science with specialization in Tribo Materials
Identifiers
URN: urn:nbn:se:uu:diva-230987DOI: 10.1016/j.surfcoat.2014.09.061ISI: 000346895000008OAI: oai:DiVA.org:uu-230987DiVA: diva2:742609
Available from: 2014-09-02 Created: 2014-09-02 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Triboactive Low-Friction Coatings Based on Sulfides and Carbides
Open this publication in new window or tab >>Triboactive Low-Friction Coatings Based on Sulfides and Carbides
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For sustainable development, it is highly important to limit the loss of energy and materials in machines used for transportation, manufacturing, and other purposes. Large improvements can be achieved by reducing friction and wear in machine elements, for example by the application of coatings. This work is focused on triboactive coatings, for which the outermost layer changes in tribological contacts to form so-called tribofilms. The coatings are deposited by magnetron sputtering (a physical vapor deposition method) and thoroughly chemically and structurally characterized, often theoretically modelled, and tribologically evaluated, to study the connection between the composition, structure and tribological performance of the coatings.

Tungsten disulfide, WS2, is a layered material with the possibility of ultra-low friction. This work presents a number of nanocomposite or amorphous coatings based on WS2, which combine the low friction with improved mechanical properties. Addition of N can give amorphous coatings consisting of a network of W, S and N with N2 molecules in nanometer-sized pockets, or lead to the formation of a metastable cubic tungsten nitride. Co-deposition with C can also give amorphous coatings, or nanocomposites with WSx grains in an amorphous C-based matrix. Further increase in coating hardness is achieved by adding both C and Ti, forming titanium carbide. All the WS2-based materials can provide very low friction (down to µ<0.02) by the formation of WS2 tribofilms, but the performance is dependent on the atmosphere as O2 and H2O can be detrimental to the tribofilm functionality.

Another possibility is to form low-friction tribofilms by tribochemical reactions between the two surfaces in contact. Addition of S to TiC/a-C nanocomposite coatings leads to the formation of a metastable S-doped carbide phase, TiCxSy, from which S can be released. This enables the formation of low-friction WS2 tribofilms when a Ti-C-S coating is run against a W counter-surface. Reduced friction, at a moderate level, also occurs for steel counter-surfaces, likely due to formation of beneficial iron sulfide tribofilms.

The studied coatings, whether based on WS2 or TiC, are thus triboactive, with the ability to form low-friction tribofilms in a sliding contact.

 

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1183
Keyword
coatings, thin films, tribology, tungsten disulfide, transition metal dichalcogenide, nanocomposite, TiC/a-C, tribofilms, PVD, XPS, HAXPES, XRD, SEM, TEM, Raman spectroscopy, nanoindentation
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-230989 (URN)978-91-554-9041-6 (ISBN)
Public defence
2014-10-31, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-10-09 Created: 2014-09-02 Last updated: 2015-01-23Bibliographically approved
2. Formation and Function of Low-Friction Tribofilms
Open this publication in new window or tab >>Formation and Function of Low-Friction Tribofilms
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of low-friction coatings on machine elements is steadily increasing, and they are expected to play an important role in the reduction of fuel consumption of future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribofilms, which then cover the coating surface and its counter surface. It is within these tribofilms that sliding takes place, and their properties largely determine the performance. The role of the coating is then not to provide low friction, but to supply support and constituents for the tribofilm.

In this thesis, the formation of such tribofilms has been studied for a number of different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, such as increased surface roughness, varied surrounding atmosphere and reduced availability of the tribofilm constituents has been given special attention.

For TaC/aC coatings, the formation of a functioning tribofilm was found to be a multi-step process, where wear fragments are formed, agglomerated, compacted and eventually stabilized into a dense film of fine grains. This formation is delayed by a moderate roughening of the coated surface.

Coatings based on tungsten disulphide (WS2) are often able to provide exceptionally low friction, but their use is restricted by their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties can be achieved by addition of for example carbon, but the achievable hardness is still limited. When titanium was added to W-S-C coatings, a carbidic hard phase was formed, causing drastically increased hardness, with retained low friction. Titanium oxides in the tribofilms however caused the friction to be high initially and unstable in the long term. In a study of W-S-N coatings, the effects of humidity and oxygen were studied separately, and it was found that the detrimental role of oxygen is larger than often assumed.

Low friction tribofilms may form by rearrangement of coating material, but also by tribochemical reactions between constituents of the coating and its counter surface. This was observed for Ti-C-S coatings, which formed WS2 tribofilms when sliding against tungsten counter surfaces, leading to dramatic friction reductions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 76 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1187
Keyword
tribology, tribofilm, PVD, coating, low-friction, tungsten disulphide, transition metal dichalcogenide, tribochemistry
National Category
Tribology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-233712 (URN)978-91-554-9065-2 (ISBN)
Public defence
2014-11-28, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-11-05 Created: 2014-10-08 Last updated: 2015-01-23

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